1-[(4-Methyl-quinazolin-2-yl)methyl]-3-methyl-7-(2-butyn-1-yl)-8-(3-(R)-amino-piperidin-1-yl)-xanthine, whose international nonproprietary name is Linagliptin, has the following structure as shown below:

Up to now, several synthetic routes to synthesize the molecule have been described by several references.
For example, a process for the preparation of Linagliptin is disclosed in the references WO 2004/018468 and WO 2006/048427.
The reference WO 2014/059938 describes a further process for preparation of Linagliptin involving a phase transfer catalyst and using protected (R)-piperidine-3-amine derivatives. The process of WO 2014/059938 requires an amine protecting group, such as tert-butyloxy carbonyl (Boc) protection and de-protection of the intermediates used in the late stage of the synthesis process.
The reference WO 2013/098775 describes a further process for preparation of Linagliptin: (R)-piperidine-3-amine (or its dihydrochloride) is said to be used directly to react with 1-[(4-methyl-quinazolin-2-yl)methyl]-3-methyl-7-(2-butyn-1-yl)-8-bromo-xanthine in the presence of a suitable base (particularly K2CO3) in an inert organic solvent (particularly DMF or MIBK) to give Linagliptin. However, the reference WO 2013/098775 only discloses to use the 1-[(4-methyl-quinazolin-2-yl)methyl]-3-methyl-7-(2-butyn-1-yl)-8-bromo-xanthine as intermediate.
It is desired to find a further process for the preparation of Linagliptin which is suitable for large scale production, particularly for pharmaceutical purposes.